They might have spider mites that are killing the needles & will kill the tree eventually. Put a piece of white paper under the branches & tap. You shoud be able to see them or wipe you hand across the paper & they will leave a smear.
Hose the tree off with as much preassure as you can then spray with an insecticide.
The dead may not grow back, but give it some time. Even until next spring.

The problem with softwood trees like spruce is that they will not "fill in" like a hardwood such as maple or oak. Once you cut/trim them they will still grow,but will not grow new branches. The top of the tree MIGHT be a different story though,but I don't think so.
If you plan on doing this yourself,I would call a local college that has a arborist program,or maybe your local DNR. They would be happy to answer any questions for FREE!